Tech Chip Guiding and Focus Adjustments
The LBC Focal Plane Array contains 2 256×2048 pixel (1 x 7.8 arcmin) technical chips on either side of the science chips (see Focal Plane Layout) which are used for guiding (tech chip #1) and maintaining focus (tech chip #2); however, since 2021 the far more accurate focus corrections from the telescope metrology system have superseded those coming from tech chip #2.
Tech Chip Guiding
Guiding is done only for exposures longer than 40 seconds. Upon starting the science exposure, and after a few second delay, an 8 second exposure is taken with tech chip #1, and it is automatically searched for stars above a threshold of 10 sigma. If no stars are found, the exposure time is doubled; this continues until at least one star is detected or the maximum guiding exposure time (64 seconds) is reached. When a number of sources are detected, additional criteria on their shape (SExtractor flag < 10), flux (between 5 and 60,000 counts) and magnitude are imposed, and they are ranked according to signal-to-noise ratio, SNR. Only stars with SNR >= 28 are used. A thumbnail image of the brightest guide star is displayed on the LBC User Interface and its FWHM reported. Note that before a guide star is selected, the thumbnail which is displayed is the latest image in the buffer; this could be from the end of the previous night. Once a new set of guide stars is found, this image will be replaced by the current star image. When no guide star is found, a question mark replaces the guide star image. For the Red camera, the guide chip is slightly out of the focal plane and the observer should expect the reported FWHM to be larger than for the science images, by about 0.2″.
♦The fact that the technical chips are fixed in the focal plane and are behind the same shutter and filter as the science chips leads to some characteristics of LBC guiding that the proposer/observer should be aware of:
- Brighter stars will be needed for observations through lower throughput filters: the U filters (U-Bessel, SDT_Uspec), Y-FAN and the intermediate band TiO, CN and F972N20 filters;
- Guiding can only be started after the science exposure has begun;
- Guiding is relative to the first guide image during any given exposure. After a dither of any size, a new guide reference position is set. This means that during any given observation consisting of multiple exposures, the science field will drift on the array.
Binocular Guiding
During binocular observations the guide corrections are automatically optimized to offload as much as possible to each primary mirror before correcting the mount. Given that the technical chips are behind the shutters of each channel, and that both channels may not be exposing in sync, it is probable that at times only one side will be guiding. This has not produced any issues with image quality.
Tech Chip Focusing
The telescope metrology system, when it is running, more accurately measures focus changes and therefore overrides the tech chip focus corrections described here. The information below has been kept for completeness.
Technical chip 2, which is used for image analysis, has been positioned 0.8 mm below the focal plane to produce extra-focal pupil images which are automatically stacked and analyzed to yield focus corrections which are sent to the primary mirror once the exposure is over and the shutter has been closed. This correction is relative to first stacked exposure in that filter, so if you only take one science exposure per filter before dithering no correction is made. Both technical chips must use the same integration time, and the integration time on tech chip 2 is determined by that required by the guide star selection process. The image analysis works automatically and currently does not produce obvious feedback to the observer. To verify that it is running and sending corrections to the primary, the observer can check the value of Z04 in the LBC logfile, and the telescope OSA can monitor the value of Z4 in the PSF GUI.
